DBS of the PSA and the VIM in essential tremor

Functional Neuroimaging During Asleep DBS Surgery: A Proof of Concept Study

Object: A real-time functional magnetic resonance imaging (fMRI) feedback during ventral intermediate nucleus (VIM) deep brain stimulation (DBS) under general anesthesia (or “asleep” DBS) does not exist. We hypothesized that it was feasible to acquire a reliable and responsive fMRI during asleep VIM DBS surgery.

Methods: We prospectively enrolled 10 consecutive patients who underwent asleep DBS for the treatment of medication-refractory essential tremor. Under general anesthesia, we acquired resting-state functional MRI immediately before and after the cannula insertion. Reliability was determined by a temporal signal-to-noise-ratio >100. Responsiveness was determined based on the fMRI signal change upon insertion of the cannula to the VIM.

Results: It was feasible to acquire reliable fMRI during asleep DBS surgery. The fMRI signal was responsive to the brain cannula insertion, revealing a reduction in the tremor network's functional connectivity, which did not reach statistical significance in the group analysis.

Conclusions: It is feasible to acquire a reliable and responsive fMRI signal during asleep DBS. The acquisition steps and the preprocessing pipeline developed in these experiments will be useful for future investigations to develop fMRI-based feedback for asleep DBS surgery.

Deep Brain Stimulation for Tremor and Dystonia

Deep brain stimulation (DBS) is the most commonly used surgical treatment for drug-refractory movement disorders such as tremor and dystonia. Appropriate patient selection along with target selection is important to ensure optimal outcome without complications. This review summarizes the recent literature regarding the mechanism of action, indications, outcome, and complications of DBS in tremor and dystonia. A comparison with other modalities of surgical interventions is discussed along with a note of the recent advances in technology. Future research needs to be directed to understand the underlying etiopathogenesis of the disease and the way in which DBS modulates the intracranial abnormal networks.

Speech Function Following Deep Brain Stimulation of the Caudal Zona Incerta: Effects of Habitual and High-Amplitude Stimulation

Purpose Deep brain stimulation (DBS) is often successful in alleviating motor symptoms of essential tremor (ET); however, DBS may also induce adverse speech effects. The caudal zona incerta (cZi) is a promising DBS target for tremor, but less is known about the consequences of cZi DBS for speech. This preliminary study examined how habitual cZi DBS and cZi stimulation at high amplitudes may affect speech function in persons with ET. Method Fourteen participants with ET were evaluated: off stimulation, on habitual cZi DBS, and with unilateral cZi stimulation at increasing stimulation amplitudes. At each stimulation condition, the participants read three 16-word sentences. Two speech-language pathologists made audio-perceptual consensus ratings of overall speech function, articulation, and voice using a visual sort and rate method. Rated functions when off stimulation, on habitual cZi DBS, and at maximal-amplitude stimulation were compared using Friedman nonparametric tests. For participants with bilateral habitual DBS (n = 5), the effects of bilateral and unilateral stimulation were described in qualitative terms. Results Habitual cZi DBS had no significant group-level effect on any of the investigated speech parameters. Maximal-amplitude stimulation had a small but significant negative effect on articulation. Participants with reduced articulatory precision (n = 9) had more medially placed electrodes than the nonaffected group (n = 5). Bilateral and unilateral left stimulation had comparable effects on speech. Conclusions Findings from this preliminary study of cZi DBS indicate that speech is generally not affected by stimulation at habitual levels. High-amplitude cZi stimulation may, however, induce adverse effects, particularly on articulation. Instances of decreased articulatory function were associated with stimulation of more medial electrode contacts, which could suggest cerebello-rubrospinal involvement.

Voxel-Based Morphometry of Cerebellar Lobules in Essential Tremor

BACKGROUND

The extent of neurodegeneration underlying essential tremor (ET) remains a matter of debate. Despite various extents of cerebellar atrophy on structural magnetic resonance imaging (MRI), previous studies have shown substantial heterogeneity and included a limited number of patients. Novel automated pipelines allow detailed segmentation of cerebellar lobules based on structural MRI.

OBJECTIVE

To compare the volumes of cerebellar lobules in ET patients with those in healthy controls (HCs) using an automated segmentation pipeline.

METHODS

Structural MRI scans of ET patients eligible for deep brain stimulation (n = 55) and of age-matched and gender-matched HCs (n = 55, from the IXI database) were segmented using the automated CEREbellum Segmentation pipeline. Lobule-specific volume differences between the ET and HC groups were evaluated using a general linear model corrected for multiple tests.

RESULTS

Total brain tissue volumes did not differ between the ET and HC groups. ET patients demonstrated reduced volumes of lobules I-II, left Crus II, left VIIB, and an increased volume of right X when compared with the HC group.

CONCLUSION

A large cohort of ET patients demonstrated subtle signs of decreased cerebellar lobule volumes. These findings oppose the hypothesis of localized atrophy in cerebellar motor areas in ET, but not the possibility of cerebellar pathophysiology in ET. Prospective investigations using alternative neuroimaging modalities may further elucidate the pathophysiology of ET and provide insights into diagnostic and therapeutic approaches.

Stimulation-Induced Side Effects of Deep Brain Stimulation in the Ventralis Intermedius and Posterior Subthalamic Area for Essential Tremor

Deep brain stimulation (DBS) targeting the ventralis intermedius (VIM) nucleus of the thalamus and the posterior subthalamic area (PSA) has been shown to be an effective treatment for essential tremor (ET). The aim of this study was to compare the stimulation-induced side effects of DBS targeting the VIM and PSA using a single electrode. Patients with medication-refractory ET who underwent DBS electrode implantation between July 2011 and October 2020 using a surgical technique that simultaneously targets the VIM and PSA with a single electrode were enrolled in this study. A total of 93 patients with ET who had 115 implanted DBS electrodes (71 unilateral and 22 bilateral) were enrolled. The Clinical Rating Scale for Tremor (CRST) subscores improved from 20.0 preoperatively to 4.3 (78.5% reduction) at 6 months, 6.3 (68.5% reduction) at 1 year, and 6.5 (67.5% reduction) at 2 years postoperation. The best clinical effect was achieved in the PSA at significantly lower stimulation amplitudes. Gait disturbance and clumsiness in the leg was found in 13 patients (14.0%) upon stimulation of the PSA and in significantly few patients upon stimulation of the VIM (p = 0.0002). Fourteen patients (15.1%) experienced dysarthria when the VIM was stimulated; this number was significantly more than that with PSA stimulation (p = 0.0233). Transient paresthesia occurred in 13 patients (14.0%) after PSA stimulation and in six patients (6.5%) after VIM stimulation. Gait disturbance and dysarthria were significantly more prevalent in patients undergoing bilateral DBS than in those undergoing unilateral DBS (p = 0.00112 and p = 0.0011, respectively). Paresthesia resolved either after reducing the amplitude or switching to bipolar stimulation. However, to control gait disturbance and dysarthria, some loss of optimal tremor control was necessary at that particular electrode contact. In the present study, the most common stimulation-induced side effect associated with VIM DBS was dysarthria, while that associated with PSA DBS was gait disturbance. Significantly, more side effects were associated with bilateral DBS than with unilateral DBS. Therefore, changing active DBS contacts to simultaneous targeting of the VIM and PSA may be especially helpful for ameliorating stimulation-induced side effects.

Wearable Peripheral Electrical Stimulation Devices for the Reduction of Essential Tremor: A Review

Essential tremor is the most common pathological tremor, with a prevalence of 6.3% in people over 65 years of age. This disorder interferes with a patient's ability to carry out activities of daily living independently, and treatment with medical and surgical interventions is often insufficient or contraindicated. Mechanical orthoses have not been widely adopted by patients due to discomfort and lack of discretion. Over the past 30 years, peripheral electrical stimulation has been investigated as a possible treatment for patients who have not found other treatment options to be satisfactory, with wearable devices revolutionizing this emerging approach in recent years. In this paper, an overview of essential tremor and its current medical and surgical treatment options are presented. Following this, tremor detection, measurement and characterization methods are explored with a focus on the measurement options that can be incorporated into wearable devices. Then, novel interventions for essential tremor are described, with a detailed review of open and closed-loop peripheral electrical stimulation methods. Finally, discussion of the need for wearable closed-loop peripheral electrical stimulation devices for essential tremor, approaches in their implementation, and gaps in the literature for further research are presented.

Comparative connectivity correlates of dystonic and essential tremor deep brain stimulation

The pathophysiology of dystonic tremor and essential tremor remains partially understood. In patients with medication-refractory dystonic tremor or essential tremor, deep brain stimulation (DBS) targeting the thalamus or posterior subthalamic area has evolved into a promising treatment option. However, the optimal DBS targets for these disorders remains unknown. This retrospective study explored the optimal targets for DBS in essential tremor and dystonic tremor using a combination of volumes of tissue activated estimation and functional and structural connectivity analyses. We included 20 patients with dystonic tremor who underwent unilateral thalamic DBS, along with a matched cohort of 20 patients with essential tremor DBS. Tremor severity was assessed preoperatively and approximately 6 months after DBS implantation using the Fahn-Tolosa-Marin Tremor Rating Scale. The tremor-suppressing effects of DBS were estimated using the percentage improvement in the unilateral tremor-rating scale score contralateral to the side of implantation. The optimal stimulation region, based on the cluster centre of gravity for peak contralateral motor score improvement, for essential tremor was located in the ventral intermediate nucleus region and for dystonic tremor in the ventralis oralis posterior nucleus region along the ventral intermediate nucleus/ventralis oralis posterior nucleus border (4 mm anterior and 3 mm superior to that for essential tremor). Both disorders showed similar functional connectivity patterns: a positive correlation between tremor improvement and involvement of the primary sensorimotor, secondary motor and associative prefrontal regions. Tremor improvement, however, was tightly correlated with the primary sensorimotor regions in essential tremor, whereas in dystonic tremor, the correlation was tighter with the premotor and prefrontal regions. The dentato-rubro-thalamic tract, comprising the decussating and non-decussating fibres, significantly correlated with tremor improvement in both dystonic and essential tremor. In contrast, the pallidothalamic tracts, which primarily project to the ventralis oralis posterior nucleus region, significantly correlated with tremor improvement only in dystonic tremor. Our findings support the hypothesis that the pathophysiology underpinning dystonic tremor involves both the cerebello-thalamo-cortical network and the basal ganglia-thalamo-cortical network. Further our data suggest that the pathophysiology of essential tremor is primarily attributable to the abnormalities within the cerebello-thalamo-cortical network. We conclude that the ventral intermediate nucleus/ventralis oralis posterior nucleus border and ventral intermediate nucleus region may be a reasonable DBS target for patients with medication-refractory dystonic tremor and essential tremor, respectively. Uncovering the pathophysiology of these disorders may in the future aid in further improving DBS outcomes.

Selecting the Most Effective DBS Contact in Essential Tremor Patients Based on Individual Tractography

Postoperative choice of the most effective deep brain stimulation (DBS) contact in patients with essential tremor (ET) so far relies on lengthy clinical testing. Previous studies showed that the postoperative effectiveness of DBS contacts depends on the distance to the dentatorubrothalamic tract (DRTT). Here, we investigated whether the most effective DBS contact could be determined from calculating stimulation overlap with the individual DRTT. Seven ET patients with bilateral thalamic deep brain stimulation were included retrospectively. Tremor control was assessed for each contact during test stimulation with 2mA. Individual DRTTs were identified from diffusion tensor imaging and contacts were ranked by their stimulation overlap with the respective DRTT in relation to their clinical effectiveness. A linear mixed-effects model was calculated to determine the influence of the DRTT overlap on tremor control. In 92.9% of investigated DBS leads, the contact with the best clinical effect was the contact with the highest or second-highest DRTT-overlap. At the group level, the DRTT-overlap explained 26.7% of the variance in the clinical outcomes (p < 0.001). Our data suggest that the overlap with the DRTT based on individual tractography may serve as a marker to determine the most effective DBS contact in ET patients and reduce burdensome clinical testing in the future.

Deep-Brain Stimulation for Essential Tremor and Other Tremor Syndromes: A Narrative Review of Current Targets and Clinical Outcomes

Tremor is a prevalent symptom associated with multiple conditions, including essential tremor (ET), Parkinson's disease (PD), multiple sclerosis (MS), stroke and trauma. The surgical management of tremor evolved from stereotactic lesions to deep-brain stimulation (DBS), which allowed safe and reversible interference with specific neural networks. This paper reviews the current literature on DBS for tremor, starting with a detailed discussion of current tremor targets (ventral intermediate nucleus of the thalamus (Vim), prelemniscal radiations (Raprl), caudal zona incerta (Zi), thalamus (Vo) and subthalamic nucleus (STN)) and continuing with a discussion of results obtained when performing DBS in the various aforementioned tremor syndromes. Future directions for DBS research are then briefly discussed.

Revisiting Forel Field Surgery

BACKGROUND

Lesioning the Forel field or the subthalamic region is considered a possible treatment for tremoric patients with Parkinson disease, essential tremor, and other diseases. This surgical treatment was performed in the 1960s to 1970s and was an alternative to thalamotomy. Recently, there has been increasing interest in the reappraisal of stimulating and/or lesioning these targets, partly as a result of innovations in imaging and noninvasive ablative technologies, such as magnetic resonance-guided focused ultrasonography.

OBJECTIVE

We wanted to perform a thorough review of the subthalamic region, both from an anatomic and a surgical standpoint, to offer a comprehensive and updated analysis of the techniques and results reported for patients with tremor treated with different techniques.

METHODS

We performed a systematic review of the literature, gathering articles that included patients who underwent ablative or stimulation surgical techniques, targeting the pallidothalamic pathways (pallidothalamic tractotomy), cerebellothalamic pathway (cerebellothalamic tractotomy), or subthalamic area.

RESULTS

Pallidothalamic tractotomy consists of a reduced area that includes pallidofugal pathways. It may be considered an interesting target, given the benefit/risk ratio and the clinical effect, which, compared with pallidotomy, involves a lower risk of injury or involvement of vital structures such as the internal capsule or optic tract. Cerebellothalamic tractotomy and/or posterior subthalamic area are other alternative targets to thalamic stimulation or ablative surgery.

CONCLUSIONS

Based on the significant breakthrough that magnetic resonance-guided focused ultrasonography has meant in the neurosurgical world, some classic targets such as the pallidothalamic tract, Forel field, and posterior subthalamic area may be reconsidered as surgical alternatives for patients with movement disorders.

Network Fingerprint of Stimulation-Induced Speech Impairment in Essential Tremor

OBJECTIVE

This study was undertaken to gain insights into structural networks associated with stimulation-induced dysarthria (SID) and to predict stimulation-induced worsening of intelligibility in essential tremor patients with bilateral thalamic deep brain stimulation (DBS).

METHODS

Monopolar reviews were conducted in 14 essential tremor patients. Testing included determination of SID thresholds, intelligibility ratings, and a fast syllable repetition task. Volumes of tissue activated (VTAs) were calculated to identify discriminative fibers for stimulation-induced worsening of intelligibility in a structural connectome. The resulting fiber-based atlas structure was then validated in a leave-one-out design.

RESULTS

Fibers determined as discriminative for stimulation-induced worsening of intelligibility were mainly connected to the ipsilateral precentral gyrus as well as to both cerebellar hemispheres and the ipsilateral brain stem. In the thalamic area, they ran laterally to the thalamus and posteromedially to the subthalamic nucleus, in close proximity, mainly anterolaterally, to fibers beneficial for tremor control as published by Al-Fatly et al in 2019. The overlap of the respective clinical stimulation setting's VTAs with these fibers explained 62.4% (p < 0.001) of the variance of stimulation-induced change in intelligibility in a leave-one-out analysis.

INTERPRETATION

This study demonstrates that SID in essential tremor patients is associated with both motor cortex and cerebellar connectivity. Furthermore, the identified fiber-based atlas structure might contribute to future postoperative programming strategies to achieve optimal tremor control without speech impairment in essential tremor patients with thalamic DBS. ANN NEUROL 2021;89:315-326.

Endurance of Short Pulse Width Thalamic Stimulation Efficacy in Intention Tremor

The benefit of short pulse width stimulation in patients suffering from essential tremor (ET) refractory to thalamic deep brain stimulation remains controversial. Here, we add to the minimal body of evidence available by reporting the effect of this type of stimulation in 3 patients with a persistent and severe intention tremor component despite iterative DBS setting adjustments. While a reduction in pulse width to 30 μs initially showed promise in these patients by improving tremor control and mitigating cerebellar side effects arguably by widening the therapeutic window, these benefits seemed to dissipate during early follow-up. Our experience supports the need for measuring longer-term outcomes when reporting the usefulness of this mode of stimulation in ET.

Advances and Future Directions of Neuromodulation in Neurologic Disorders

"Deep brain stimulation is a safe and effective therapy for the management of a variety of neurologic conditions with Food and Drug Administration or humanitarian exception approval for Parkinson disease, dystonia, tremor, and obsessive-compulsive disorder. Advances in neurophysiology, neuroimaging, and technology have driven increasing interest in the potential benefits of neurostimulation in other neuropsychiatric conditions including dementia, depression, pain, Tourette syndrome, and epilepsy, among others. New anatomic or combined targets are being investigated in these conditions to improve symptoms refractory to medications or standard stimulation."

Structure-function relationship of the posterior subthalamic area with directional deep brain stimulation for essential tremor

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Directional DBS of the DRTT and the zona incerta is correlated with tremor suppression.

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Activation patterns for tremor suppression and side effects involve mostly the dentato-rubro-thalamic tract and the zona incerta.

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Concomitant side effects often limit the therapeutic window of directional deep brain stimulation.

Deep Brain Stimulation of the posterior subthalamic area is an emergent target for the treatment of Essential Tremor. Due to the heterogeneous and complex anatomy of the posterior subthalamic area, it remains unclear which specific structures mediate tremor suppression and different side effects. The objective of the current work was to yield a better understanding of what anatomical structures mediate the different clinical effects observed during directional deep brain stimulation of that area.

We analysed a consecutive series of 12 essential tremor patients. Imaging analysis and systematic clinical testing performed 4–6 months postoperatively yielded location, clinical efficacy and corresponding therapeutic windows for 160 directional contacts. Overlap ratios between individual activation volumes and neighbouring thalamic and subthalamic nuclei as well as individual fiber tracts were calculated. Further, we generated stimulation heatmaps to assess the area of activity and structures stimulated during tremor suppression and occurrence of side effects.

Stimulation of the dentato-rubro-thalamic tract and the zona incerta was most consistently correlated with tremor suppression. Both individual and group analysis demonstrated a similar pattern of activation for tremor suppression and different sorts of side-effects. Unlike current clinical concepts, induction of spasms and paresthesia were not correlated with stimulation of the corticospinal tract and the medial lemniscus. Furthermore, we noticed a significant difference in the therapeutic window between the best and worst directional contacts. The best directional contacts did not provide significantly larger therapeutic windows than omnidirectional stimulation at the same level.

Deep brain stimulation of the posterior subthalamic area effectively suppresses all aspects of ET but can be associated with concomitant side effects limiting the therapeutic window. Activation patterns for tremor suppression and side effects were similar and predominantly involved the dentato-rubro-thalamic tract and the zona incerta. We found no different activation patterns between different types of side effects and no clear correlation between structure and function. Future studies with use of more sophisticated modelling of activation volumes taking into account fiber heterogeneity and orientation may eventually better delineate these different clusters, which may allow for a refined targeting and programming within this area.

Deep brain stimulation and other surgical modalities for the management of essential tremor

INTRODUCTION

Surgical treatments are considered for essential tremor (ET) when patients do not respond to oral pharmacological therapies. These treatments mainly comprise radiofrequency (RF) thalamotomy, gamma knife radiosurgery (GKRS), deep brain stimulation (DBS), and focused ultrasound (FUS) procedures.

AREAS COVERED

We reviewed the strengths and weaknesses of each procedure and clinical outcomes for 7 RF studies (n = 85), 11 GKRS (n = 477), 33 DBS (n = 1061), and 13 FUS studies (n = 368). A formal comparison was not possible given the heterogeneity in studies. Improvements were about 42%-90% RF, 10%-79% GKRS, 45%-83% DBS, 42%-83% FUS at short-term follow-up (<12 months) and were about 54%-82% RF, 11%-84% GKRS, 18%-92% DBS, and 42%-80% FUS at long-term follow-up (>12 months).

EXPERT OPINION

We found DBS with inherent advantages of being an adjustable and reversible procedure as the most frequently employed surgical procedure for control of ET symptoms. FUS is a promising procedure but has limited applicability for unilateral control of symptoms. RF is invasive, and GKRS has unpredictable delayed effects. Each of these surgical modalities has advantages and limitations that need consideration when selecting a treatment for the ET patients.

Managing Essential Tremor

Essential tremor is one of the most common tremor syndromes. According to the recent tremor classification, tremor as a symptom is defined as an involuntary, rhythmic, oscillatory movement of a body part and is classified along two axes: axis 1-defining syndromes based on the clinical features such as historical features, tremor characteristics, associated signs, and laboratory tests; and axis 2-classifying the etiology (Bhatia et al., Mov Disord 33:75-87, 2018). The management of this condition has two major approaches. The first is to exclude treatable etiologies, as particularly during the onset of this condition the presentation of a variety of etiologies can be with monosymptomatic tremor. Once the few etiologies with causal treatments are excluded, all further treatment is symptomatic. Shared decision-making with enabling the patient to knowledgeably choose treatment options is needed to customize the management. Mild to moderate tremor severity can sometimes be controlled with occupational treatment, speech therapy of psychotherapy, or adaptation of coping strategy. First-line pharmacological treatments include symptomatic treatment with propranolol, primidone, and topiramate. Botulinum toxin is for selected cases. Invasive treatments for essential tremor should be considered for severe tremors. They are generally accepted as the most powerful interventions and provide not only improvement of tremor but also a significant improvement of life quality. The current standard is deep brain stimulation (DBS) of the thalamic and subthalamic region. Focused ultrasound thalamotomy is a new therapy attracting increasing interest. Radiofrequency lesioning is only rarely done if DBS or focused ultrasound is not possible. Radiosurgery is not well established. We present our treatment algorithm.

Connectivity profile of thalamic deep brain stimulation to effectively treat essential tremor

Essential tremor is the most prevalent movement disorder and is often refractory to medical treatment. Deep brain stimulation offers a therapeutic approach that can efficiently control tremor symptoms. Several deep brain stimulation targets (ventral intermediate nucleus, zona incerta, posterior subthalamic area) have been discussed for tremor treatment. Effective deep brain stimulation therapy for tremor critically involves optimal targeting to modulate the tremor network. This could potentially become more robust and precise by using state-of-the-art brain connectivity measurements. In the current study, we used two normative brain connectomes (structural and functional) to show the pattern of effective deep brain stimulation electrode connectivity in 36 patients with essential tremor. Our structural and functional connectivity models were significantly predictive of postoperative tremor improvement in out-of-sample data (P < 0.001 for both structural and functional leave-one-out cross-validation). Additionally, we segregated the somatotopic brain network based on head and hand tremor scores. These resulted in segregations that mapped onto the well-known somatotopic maps of both motor cortex and cerebellum. Crucially, this shows that slightly distinct networks need to be modulated to ameliorate head versus hand tremor and that those networks could be identified based on somatotopic zones in motor cortex and cerebellum. Finally, we propose a multi-modal connectomic deep brain stimulation sweet spot that may serve as a reference to enhance clinical care, in the future. This spot resided in the posterior subthalamic area, encroaching on the inferior borders of ventral intermediate nucleus and sensory thalamus. Our results underscore the importance of integrating brain connectivity in optimizing deep brain stimulation targeting for essential tremor.

A Custom Made Electrode Construct and Reliable Implantation Method That Allows for Long-Term Bilateral Deep Brain Stimulation in Mice

Objectives

The underlying mechanisms behind the therapeutic and side effects of deep brain stimulation (DBS) need further investigation. The utilization of transgenic mouse lines is a suitable approach to better understand the cellular and network effects of DBS. However, not many bilateral DBS studies have been conducted in mice. This might be due to a lack of commercially available bilateral DBS constructs.

Materials and Methods

We developed an approach to perform repetitive long‐term DBS in freely moving mice. In this study, we implanted an in‐house custom‐made DBS construct containing two bipolar concentric electrodes to target the subthalamic nucleus (STN) bilaterally. Subsequently, we stimulated half of the animals with clinically relevant parameters three to five times a week with a duration of 20 min for ten weeks. Several behavioral tests were conducted of which the open field test (OFT) is shown to validate the reliability of this electrode construct and implantation method. Furthermore, we performed fiber photometry measurements to show the acute effect of STN‐DBS on serotonin network activity in the dorsal raphe nucleus.

Results

Repetitive DBS and long‐term behavioral testing were performed without complications. STN‐DBS resulted in an increase of the distance traveled in the OFT and a reduction of calcium levels in serotonergic neurons of the dorsal raphe nucleus. None of the mice had lost their electrodes and postmortem evaluation of the tissue showed accurate targeting of the STN without excessive gliosis.

Conclusion

The DBS electrode construct and implantation method described can be used for long‐term DBS studies to further investigate the mechanisms underlying DBS.

Intraoperative Evaluation of Essential Vocal Tremor in Deep Brain Stimulation Surgery

Purpose Essential vocal tremor (EVT) is a prevalent and difficult-to-manage voice disorder. There is evidence that deep brain stimulation (DBS) of the ventral intermediate nucleus (Vim) of the thalamus may be beneficial for treating EVT. The objective of this preliminary investigation was to conduct intraoperative voice assessments during Vim-DBS implantation in order to evaluate immediate voice outcomes in medication-refractory essential tremor patients with co-occurring EVT. Method Seven adult subjects diagnosed with EVT undergoing Vim-DBS surgery participated in this investigation. Voice samples of sustained vowels were collected by a speech-language pathologist preoperatively and intraoperatively, immediately following Vim-DBS electrode placement. Voice evaluation included objective acoustic assessment of the rate and extent of EVT fundamental frequency and intensity modulation and subjective perceptual ratings of EVT severity. Results The rate of intensity modulation, extent of fundamental frequency modulation, and perceptual rating of EVT severity were significantly reduced intraoperatively as compared to preoperatively. Moderate, positive correlations were appreciated between a subset of acoustic measures and perceptual severity ratings. Conclusions The results of this study demonstrate a speech-language pathologist can conduct intra-operative evaluation of EVT during DBS surgery. Using a noninvasive, simple acoustic recording method, we were able to supplement perceptual subjective observation with objective assessment and demonstrate immediate, intraoperative improvements in EVT. The findings of this analysis inform the added value of intraoperative voice evaluation in Vim-DBS patients and contribute to the growing body of literature seeking to evaluate the efficacy of DBS as a treatment for EVT.

DBS dysfunction mimicking transient ischemic attacks-a case report

We report on a patient with thalamic deep brain stimulation (DBS) for essential tremor who was admitted to a stroke unit with transient vertigo, dysarthria, and gait disturbance. Transient ischemic attacks were assumed but fluctuating neurological symptoms persisted until presentation to a DBS center. Here, unstable high monopolar impedances of the right-hemispheric electrode contacts were detected. Surgical revision revealed a fracture of the pocket adaptor connecting this electrode to the impulse generator. Replacement resulted in stable impedances and remitted the transient neurological symptoms. Emergency and stroke doctors should be aware of neurological symptoms induced by technical dysfunctions in DBS.

A Pilot Double-Blind Randomized Trial of Perampanel for Essential Tremor

Background

Perampanel is a noncompetitive antagonist of alpha-amino-3-hydroxy-5-methylisoxazole propionic acid glutamate receptors suggested to modulate tremor.

Objectives

To assess the efficacy and tolerability of perampanel for essential tremor.

Methods

This was a double-blind, placebo-controlled, randomized, cross-over trial involving 26 patients titrated to 8 mg/day or a lower maximally tolerated dose as monotherapy or adjunct to antitremor medication. Tremor was assessed at the beginning and end of each 14-week treatment arm. The primary endpoint was change in the videotaped performance subscale of The Essential Tremor Rating Assessment Scale, scored by a blinded rater. Secondary endpoints included change in The Essential Tremor Rating Assessment Scale Activity of Daily Living and Quality of Life in Essential Tremor and Subject Global Impression of Change subscales.

Results

Data are available for 15 and 11 participants who completed placebo and perampanel arms, respectively. Perampanel was superior to placebo on the primary endpoint (P = 0.028), Activity of Daily Living (P = 0.009), and Subject Global Impression of Change (P = 0.016), but not Quality of Life (p = 0.48). Video scores were rated >50% improved in 3/11 on perampanel and 0/15 on placebo. Adverse events were more likely on perampanel (especially at >4 mg/day) than on placebo, leading to withdrawal (36% vs. 10%) and dose reduction (41% vs. 15%). Adverse events more common with perampanel included imbalance/falls (50% vs. 10%), dizziness (36% vs. 10%), and irritability (27% vs. 5%).

Conclusions

These findings suggest that perampanel exerts efficacy for some persons with essential tremor, but this population appears prone to adverse events.

Purkinje cell misfiring generates high-amplitude action tremors that are corrected by cerebellar deep brain stimulation

Tremor is currently ranked as the most common movement disorder. The brain regions and neural signals that initiate the debilitating shakiness of different body parts remain unclear. Here, we found that genetically silencing cerebellar Purkinje cell output blocked tremor in mice that were given the tremorgenic drug harmaline. We show in awake behaving mice that the onset of tremor is coincident with rhythmic Purkinje cell firing, which alters the activity of their target cerebellar nuclei cells. We mimic the tremorgenic action of the drug with optogenetics and present evidence that highly patterned Purkinje cell activity drives a powerful tremor in otherwise normal mice. Modulating the altered activity with deep brain stimulation directed to the Purkinje cell output in the cerebellar nuclei reduced tremor in freely moving mice. Together, the data implicate Purkinje cell connectivity as a neural substrate for tremor and a gateway for signals that mediate the disease.

PSA and VIM DBS efficiency in essential tremor depends on distance to the dentatorubrothalamic tract

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Proof that proximity to the dentatorubrothalamic tract (DRTT) is responsible for increased tremor suppression in deep brain stimulation (DBS) for essential tremor (ET).

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High-quality prospective, randomized, double-blind clinical data from 13 ET subjects.

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Population-based DRTTs using probabilistic tractography in state-of-the-art diffusion MRI data from the human connectome project with supplementary validation against clinical dMRI data from ET patients.

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Implications for the future of direct DBS targeting in tremor patients.

Objective

To investigate the relation between deep brain stimulation (DBS) of the posterior-subthalamic-area (PSA) and the ventral-intermediate-nucleus (VIM) and the distance to the dentatorubrothalamic tract (DRTT) in essential tremor (ET).

Methods

Tremor rating scale (TRS) hemi-scores were analyzed in 13 ET patients, stimulated in both the VIM and the PSA in a randomized, crossover trial. Distances of PSA and VIM contacts to population-based DRTTs were calculated. The relationships between distance to DRTT and stimulation amplitude, as well as DBS efficiency (TRS improvement per amplitude) were investigated.

Results

PSA contacts were closer to the DRTT (p = 0.019) and led to a greater improvement in TRS hemi-scores (p = 0.005) than VIM contacts. Proximity to the DRTT was related to lower amplitudes (p < 0.001) and higher DBS efficiency (p = 0.017).

Conclusions

Differences in tremor outcome and stimulation parameters between contacts in the PSA and the VIM can be explained by their different distance to the DRTT.

Deep brain stimulation in essential tremor: targets, technology, and a comprehensive review of clinical outcomes

Introduction: Essential tremor (ET) is a common movement disorder with an estimated prevalence of 0.9% worldwide. Deep brain stimulation (DBS) is an established therapy for medication refractory and debilitating tremor. With the arrival of next generation technology, the implementation and delivery of DBS has been rapidly evolving. This review will highlight the current applications and constraints for DBS in ET.Areas covered: The mechanism of action, targets for neuromodulation, next generation guidance techniques, symptom-specific applications, and long-term efficacy will be reviewed.Expert opinion: The posterior subthalamic area and zona incerta are alternative targets to thalamic DBS in ET. However, they may be associated with additional stimulation-induced side effects. Novel stimulation paradigms and segmented electrodes provide innovative approaches to DBS programming and stimulation-induced side effects.

The Effects of Deep Brain Stimulation on Speech Intelligibility in Persons With Essential Tremor

Purpose The aim of this study was to investigate how deep brain stimulation (DBS) of the caudal zona incerta (cZi) affects speech intelligibility in persons with essential tremor. Method Thirty-five participants were evaluated: off stimulation, on chronic stimulation optimized to alleviate tremor, and during unilateral stimulation at increasing amplitude levels. At each stimulation condition, the participants read 10 unique nonsense sentences from the Swedish Test of Intelligibility. Two listeners, blinded to stimulation condition, transcribed all recorded sentences orthographically in a randomized procedure. A mean speech intelligibility score for each patient and stimulation condition was computed, and comparisons were made between scores off and on stimulation. Results Chronic cZi-DBS had no significant effect on speech intelligibility, and there was no difference in outcome between bilateral and unilateral treatments. During unilateral stimulation at increasing amplitudes, nine participants demonstrated deteriorating speech intelligibility. These nine participants were on average older and had more superior contacts activated during the evaluation compared with the participants without deterioration. Conclusions Chronic cZi-DBS, optimized for tremor suppression, does not generally affect speech intelligibility in persons with essential tremor. Furthermore, speech intelligibility may be preserved in many individuals, even when stimulated at high amplitudes. Adverse effects of high-amplitude unilateral stimulation observed in this study were associated with stimulation originating from a more superior location, as well as with the participants' age. These results, highlighting age and stimulation location as contributing to speech intelligibility outcomes, were, however, based on a limited number of individuals experiencing adverse effects with high-amplitude stimulation and should, therefore, be interpreted with caution.

Bilateral Posterior Subthalamic Area Deep Brain Stimulation for Essential Tremor: A Case Series

Background

Deep brain stimulation (DBS) of the posterior subthalamic area (PSA) provides a potentially effective treatment for medication-refractory essential tremor (ET).

Objective

To study the clinical benefits and adverse-event profile of bilateral PSA-DBS for refractory ET.

Methods

Seven patients with refractory ET underwent bilateral PSA-DBS surgery under general anesthesia between September 2017 and May 2018. Clinical outcome assessments, using the Essential Tremor Rating Scale, were performed at 1-, 6-, and 12-month follow-up, except for the last assessment of one patient who was followed up to 9 months. Analysis was focused on changes in patients’ motor symptoms and quality of life following surgery as well as documenting the adverse-event profile associated with the surgical PSA-DBS treatment.

Results

After surgery, patients’ motor symptoms, including upper limb tremor and head tremor, were improved by 84.2% and their quality of life by 81.25% at 1-month follow-up. The clinical benefits to patients were maintained at 6-month and last follow-up. Adverse side effects included dysarthria (n = 4), balance disorder (n = 2), and paresthesia of the right limb (n = 1). No habituation effects were observed throughout the follow-up.

Conclusion

Bilateral PSA-DBS seems to offer an effective and safe alternative treatment for medically intractable ET, warranting further research into its clinical utility, adverse-event profile, and comparative effectiveness.

Stimulation maps: visualization of results of quantitative intraoperative testing for deep brain stimulation surgery

Deep brain stimulation (DBS) is an established therapy for movement disorders such as essential tremor (ET). Positioning of the DBS lead in the patient's brain is crucial for effective treatment. Extensive evaluations of improvement and adverse effects of stimulation at different positions for various current amplitudes are performed intraoperatively. However, to choose the optimal position of the lead, the information has to be "mentally" visualized and analyzed. This paper introduces a new technique called "stimulation maps," which summarizes and visualizes the high amount of relevant data with the aim to assist in identifying the optimal DBS lead position. It combines three methods: outlines of the relevant anatomical structures, quantitative symptom evaluation, and patient-specific electric field simulations. Through this combination, each voxel in the stimulation region is assigned one value of symptom improvement, resulting in the division of stimulation region into areas with different improvement levels. This technique was applied retrospectively to five ET patients in the University Hospital in Clermont-Ferrand, France. Apart from identifying the optimal implant position, the resultant nine maps show that the highest improvement region is frequently in the posterior subthalamic area. The results demonstrate the utility of the stimulation maps in identifying the optimal implant position. Graphical abstract.

Functional disconnection of the dentate nucleus in essential tremor

Despite previous functional MRI studies on alterations within the cerebello-thalamo-cortical circuit in patients with essential tremor (ET), the specific role of disconnection of the dentate nucleus (DN), the main output cerebellar pathway, still needs clarification. In this study, we evaluated DN functional connectivity (FC) changes and their relationship with motor and non-motor symptoms in ET. We studied 25 ET patients and 26 healthy controls. Tremor severity was assessed using the Fahn-Tolosa-Marin tremor rating scale (FTM-TRS) and tremor amplitude and frequency were evaluated using kinematic techniques. Cognitive profile was assessed by montreal cognitive assessment (MoCA) and frontal assessment battery (FAB). All participants underwent a 3 T MRI protocol including resting-state blood oxygenation level dependent and diffusion tensor sequences. We used a seed-based approach to investigate DN FC and to explore the diffusion properties of cerebellar peduncles. There was significantly decreased DN FC with cortical, subcortical, and cerebellar areas in ET patients compared with healthy controls. Correlation analysis showed that: (1) the DN FC with the supplementary motor area, pre and postcentral gyri, and prefrontal cortex negatively correlated with FTM-TRS score and disease duration; (2) DN FC changes in the thalamus and caudate negatively correlated with peak tremor frequency, changes in the cerebellum positively correlated with tremor amplitude, and changes in the bilateral thalamus negatively correlated with tremor amplitude, and (3) DN FC with the associative prefrontal and parietal cortices, basal ganglia, and thalamus positively correlated with the MoCA score. Diffusion abnormalities were found in the three cerebellar peduncles, which did not correlate with clinical scores.

Deep brain stimulation in the caudal zona incerta modulates the sensorimotor cerebello-cerebral circuit in essential tremor

Essential tremor is effectively treated with deep brain stimulation (DBS), but the neural mechanisms underlying the treatment effect are poorly understood. Essential tremor is driven by a dysfunctional cerebello-thalamo-cerebral circuit resulting in pathological tremor oscillations. DBS is hypothesised to interfere with these oscillations at the stimulated target level, but it is unknown whether the stimulation modulates the activity of the cerebello-thalamo-cerebral circuit during different task states (with and without tremor) in awake essential tremor patients. To address this issue, we used functional MRI in 16 essential tremor patients chronically implanted with DBS in the caudal zona incerta. During scanning, the patients performed unilateral tremor-inducing postural holding and pointing tasks as well as rest, with contralateral stimulation turned On and Off. We show that DBS exerts both task-dependent as well as task-independent modulation of the sensorimotor cerebello-cerebral regions (p ​≤ ​0.05, FWE cluster-corrected for multiple comparisons). Task-dependent modulation (DBS ​× ​task interaction) resulted in two patterns of stimulation effects. Firstly, activity decreases (blood oxygen level-dependent signal) during tremor-inducing postural holding in the primary sensorimotor cortex and cerebellar lobule VIII, and activity increases in the supplementary motor area and cerebellar lobule V during rest (p ​≤ ​0.05, post hoc two-tailed t-test). These effects represent differences at the effector level and may reflect DBS-induced tremor reduction since the primary sensorimotor cortex, cerebellum and supplementary motor area exhibit less motor task-activity as compared to the resting condition during On stimulation. Secondly, task-independent modulation (main effect of DBS) was observed as activity increase in the lateral premotor cortex during all motor tasks, and also during rest (p ​≤ ​0.05, post hoc two-tailed t-test). This task-independent effect may mediate the therapeutic effects of DBS through the facilitation of the premotor control over the sensorimotor circuit, making it less susceptible to tremor entrainment. Our findings support the notion that DBS in essential tremor is modulating the sensorimotor cerebello-cerebral circuit, distant to the stimulated target, and illustrate the complexity of stimulation mechanisms by demonstrating task-dependent as well as task-independent actions in cerebello-cerebral regions.

Effects of ventral intermediate nucleus deep brain stimulation across multiple effectors in essential tremor

OBJECTIVE

Essential tremor (ET) prominently affects the upper-limbs during voluntary movements, but can also affect the lower-limbs, head, and chin. Although deep brain stimulation (DBS) of the ventral intermediate nucleus (VIM) of thalamus improves both clinical ratings and quantitative measures of tremor, no study has quantified effects of DBS on tremor across multiple body parts. Our objective was to quantify therapeutic effects of DBS across multiple body parts in ET.

METHODS

We performed quantitative assessment of tremor in ET patients who had DBS for at least one year. We assessed tremor on and off VIM-stimulation using triaxial accelerometers on the upper-limbs, lower-limbs, head and chin during seated and standing tasks.

RESULTS

VIM-DBS significantly reduced tremor, but there was no statistical difference in degree of tremor reduction across the measured effectors. Compared to healthy controls, ET patients treated with DBS showed significantly greater tremor power (4-8 Hz) across all effectors during seated and standing tasks.

CONCLUSIONS

VIM-DBS reduced tremor in ET patients. There was no significant difference in the degree of tremor reduction across the measured effectors.

SIGNIFICANCE

This study provides new quantitative evidence that VIM-DBS is effective at reducing tremor across multiple parts of the body.

Comparison of posterior subthalamic area deep brain stimulation for tremor using conventional landmarks versus directly targeting the dentatorubrothalamic tract with tractography

OBJECTIVE

To compare posterior subthalamic area deep brain stimulation (PSA-DBS) performed in the conventional manner against diffusion tensor imaging and tractography (DTIT)-guided lead implantation into the dentatorubrothalamic tract (DRTT).

PATIENTS AND METHODS

Double-blind, randomised study involving 34 patients with either tremor-dominant Parkinson's disease or essential tremor. Patients were randomised to Group A (DBS leads inserted using conventional landmarks) or Group B (leads guided into the DRTT using DTIT). Tremor (Fahn-Tolosa-Marin) and quality-of-life (PDQ-39) scores were evaluated 0-, 6-, 12-, 36- and 60-months after surgery.

RESULTS

PSA-DBS resulted in marked tremor reduction in both groups. However, Group B patients had significantly better arm tremor control (especially control of intention tremor), increased mobility and activities of daily living, reduced social stigma and need for social support as well as lower stimulation amplitudes and pulse widths compared to Group A patients. The better outcomes were sustained for up to 60-months from surgery. The active contacts of Group B patients were consistently closer to the centre of the DRTT than in Group A. Speech problems were more common in Group A patients.

CONCLUSION

DTIT-guided lead placement results in better and more stable tremor control and fewer adverse effects compared to lead placement in the conventional manner. This is because DTIT-guidance allows closer and more consistent placement of leads to the centre of the DRTT than conventional methods.

Tissue Response to Neural Implants: The Use of Model Systems Toward New Design Solutions of Implantable Microelectrodes

The development of implantable neuroelectrodes is advancing rapidly as these tools are becoming increasingly ubiquitous in clinical practice, especially for the treatment of traumatic and neurodegenerative disorders. Electrodes have been exploited in a wide number of neural interface devices, such as deep brain stimulation, which is one of the most successful therapies with proven efficacy in the treatment of diseases like Parkinson or epilepsy. However, one of the main caveats related to the clinical application of electrodes is the nervous tissue response at the injury site, characterized by a cascade of inflammatory events, which culminate in chronic inflammation, and, in turn, result in the failure of the implant over extended periods of time. To overcome current limitations of the most widespread macroelectrode based systems, new design strategies and the development of innovative materials with superior biocompatibility characteristics are currently being investigated. This review describes the current state of the art of in vitro, ex vivo, and in vivo models available for the study of neural tissue response to implantable microelectrodes. We particularly highlight new models with increased complexity that closely mimic in vivo scenarios and that can serve as promising alternatives to animal studies for investigation of microelectrodes in neural tissues. Additionally, we also express our view on the impact of the progress in the field of neural tissue engineering on neural implant research.

A Review of Cognitive Outcomes Across Movement Disorder Patients Undergoing Deep Brain Stimulation

Introduction: Although the benefit in motor symptoms for well-selected patients with deep brain stimulation (DBS) has been established, cognitive declines associated with DBS can produce suboptimal clinical responses. Small decrements in cognition can lead to profound effects on quality of life. The growth of indications, the expansion of surgical targets, the increasing complexity of devices, and recent changes in stimulation paradigms have all collectively drawn attention to the need for re-evaluation of DBS related cognitive outcomes.

Methods: To address the impact of cognitive changes following DBS, we performed a literature review using PubMed. We searched for articles focused on DBS and cognition. We extracted information about the disease, target, number of patients, assessment of time points, cognitive battery, and clinical outcomes. Diseases included were dystonia, Tourette syndrome (TS), essential tremor (ET), and Parkinson's disease (PD).

Results: DBS was associated with mild cognitive issues even when rigorous patient selection was employed. Dystonia studies reported stable or improved cognitive scores, however one study using reliable change indices indicated decrements in sustained attention. Additionally, DBS outcomes were convoluted with changes in medication dose, alleviation of motor symptoms, and learning effects. In the largest, prospective TS study, an improvement in attentional skills was noted, whereas smaller studies reported variable declines across several cognitive domains. Although, most studies reported stable cognitive outcomes. ET studies largely demonstrated deficits in verbal fluency, which had variable responses depending on stimulation setting. Recently, studies have focused beyond the ventral intermediate nucleus, including the post-subthalamic area and zona incerta. For PD, the cognitive results were heterogeneous, although deficits in verbal fluency were consistent and related to the micro-lesion effect.

Conclusion: Post-DBS cognitive issues can impact both motor and quality of life outcomes. The underlying pathophysiology of cognitive changes post-DBS and the identification of pathways underpinning declines will require further investigation. Future studies should employ careful methodological designs. Patient specific analyses will be helpful to differentiate the effects of medications, DBS and the underlying disease state, including disease progression. Disease progression is often an underappreciated factor that is important to post-DBS cognitive issues.

Deep Brain Stimulation for Obesity: A Review and Future Directions

The global prevalence of obesity has been steadily increasing. Although pharmacotherapy and bariatric surgeries can be useful adjuvants in the treatment of morbid obesity, they may lose long-term effectiveness. Obesity result largely from unbalanced energy homeostasis. Palatable and densely caloric foods may affect the brain overlapped circuits involved with homeostatic hypothalamus and hedonic feeding. Deep brain stimulation (DBS) consists of delivering electrical impulses to specific brain targets to modulate a disturbed neuronal network. In selected patients, DBS has been shown to be safe and effective for movement disorders. We review all the cases reports and series of patients treated with DBS for obesity using a PubMed search and will address the following obesity-related issues: (i) the hypothalamic regulation of homeostatic feeding; (ii) the reward mesolimbic circuit and hedonic feeding; (iii) basic concepts of DBS as well as the rationale for obesity treatment; (iv) perspectives and challenges in obesity DBS. The small number of cases provides preliminary evidence for the safety and the tolerability of a potential DBS approach. The ventromedial (n = 2) and lateral (n = 8) hypothalamic nuclei targets have shown mixed and disappointing outcomes. Although nucleus accumbens (n = 7) targets were more encouraging for the outcomes of body weight reduction and behavioral control for eating, there was one suicide reported after 27 months of follow-up. The authors did not attribute the suicide to DBS therapy. The identification of optimal brain targets, appropriate programming strategies and the development of novel technologies will be important as next steps to move DBS closer to a clinical application. The identification of electrical control signals may provide an opportunity for closed-loop adaptive DBS systems to address obesity. Metabolic and hormonal sensors such as glycemic levels, leptin, and ghrelin levels are candidate control signals for DBS. Focused excitation or alternatively inhibition of regions of the hypothalamus may provide better outcomes compared to non-selective DBS. Utilization of the NA delta oscillation or other physiological markers from one or multiple regions in obesity-related brain network is a promising approach. Experienced multidisciplinary team will be critical to improve the risk-benefit ratio for this approach.

Mapping stimulation-induced beneficial and adverse effects in the subthalamic area of essential tremor patients

BACKGROUND

Stimulation of the subthalamic area (STA) is an effective treatment in essential tremor patients, but limited by stimulation induced adverse effects. The aim of this study was to determine the spatial distribution of stimulus related tremor suppression, ataxia induction and paresthesia of the upper limb in the subthalamic area (STA) of essential tremor patients.

METHODS

We recruited eight patients with essential tremor in a stable postoperative condition (>3 months after surgery). Stimulation-induced effects were assessed with suprathreshold stimulation. Tremor severity was assessed with the Fahn-Tolosa-Marin tremor rating scale (TRS) and cerebellar impairment was evaluated using the international cooperative ataxia rating scale (ICARS). Patients rated paresthesia intensity with a visual analog scale. Linear regression analysis was performed to associate stereotactic coordinates with tremor, ataxia and paresthesia.

RESULTS

Suprathreshold stimulation significantly decreased tremor and elicited ataxia and paresthesia in all patients (P < 0.001). Tremor rating scale (TRS) total score was positively correlated with y-coordinates (r = 0.44, P < 0.05), i.e. anterior stimulation sites were more effective to suppress tremor. Concerning adverse effects, ataxia induction was positively correlated with z-coordinates almost reaching statistical significance (r = 0.50, P = 0.07), i.e. inferior stimulation sites elicit stronger ataxia. Furthermore, paresthesia was positively correlated with y-coordinates (r = 0.66; P < 0.01) and to a lesser degree with x-coordinates (r = 0.32; P = 0.08), i.e. posterior and lateral stimulation sites within the STA caused more paresthesia.

CONCLUSION

Antero-dorso-medial stimulation site in the STA were associated with less tremor and adverse effects in our small single-center cohort of ET patients with thalamic DBS.

A review of basal ganglia circuits and physiology: Application to deep brain stimulation

INTRODUCTION

Drawing on the seminal work of DeLong, Albin, and Young, we have now entered an era of basal ganglia neuromodulation. Understanding, re-evaluating, and leveraging the lessons learned from neuromodulation will be crucial to facilitate an increased and improved application of neuromodulation in human disease.

METHODS

We will focus on deep brain stimulation (DBS) - the most common form of basal ganglia neuromodulation - however, similar principles can apply to other neuromodulation modalities. We start with a brief review of DBS for Parkinson's disease, essential tremor, dystonia, and Tourette syndrome. We then review hallmark studies on basal ganglia circuits and electrophysiology resulting from decades of experience in neuromodulation. The organization and content of this paper follow Dr. Okun's Lecture from the 2018 Parkinsonism and Related Disorders World Congress.

RESULTS

Information gained from neuromodulation has led to an expansion of the basal ganglia rate model, an enhanced understanding of nuclei dynamics, an emerging focus on pathological oscillations, a revision of the tripartite division of the basal ganglia, and a redirected focus toward individualized symptom-specific stimulation. Though there have been many limitations of the basal ganglia "box model," the construct provided the necessary foundation to advance the field. We now understand that information in the basal ganglia is encoded through complex neural responses that can be reliably measured and used to infer disease states for clinical translation.

CONCLUSIONS

Our deepened understanding of basal ganglia physiology will drive new neuromodulation strategies such as adaptive DBS or cell-specific neuromodulation through the use of optogenetics.

Unstable impedance of a single electrode contact resulting in loss of DBS therapy-a case report

Open and short circuits of electrode contacts are important technical dysfunctions of DBS. Here, we report on another type of dysfunction restricted to a single electrode contact: impedance instability within regular absolute values. After 9-year subthalamic DBS, a Parkinson patient developed unilateral motor symptoms and intermittent dysaesthesia due to impedance instability of the active contact. DBS efficacy could be restored without surgical revision by activation of the neighboring contact. During 3-year-follow-up, impedances of the dysfunctional contact varied between 1 and 3 kΩ whereas the other three contacts remained stable. Impedance documentation is crucial to identify such dysfunctions.